1. Field
The present disclosure relates to wireless communication, and more particularly, to a method and apparatus for transmitting an HARQ ACK/NACK.
2. Discussion of the Background
Automatic repeat request (ARQ) is one of the schemes that enhance reliability of a wireless communication. The ARQ refers to a scheme in which a transmitter retransmits a data signal if a data signal reception is failed at a receiver. Further, there is a scheme, hybrid automatic repeat request (HARQ), which is a combination of Forward Error Correction (FEC) and ARQ. A receiver that utilizes HARQ generally attempts an error correction for a received data signal and determines whether a retransmission needs to be performed by using an error detection code. As the error detection code, Cyclic Redundancy Check (CRC) scheme may be used. If data signal error is not detected from the detection process of CRC scheme, the receiver determines that a decoding process for the data signal is successful. In this case, the receiver transmits an Acknowledgement (ACK) signal to a transmitter. If data signal error is detected from the detection process of CRC scheme, the receiver determines that a decoding process for the data signal is not successful. In this case, the receiver transmits a Not-Acknowledgement (NACK) signal to a transmitter. If the transmitter receives the NACK signal, the transmitter may retransmit the data signal.
A wireless communication system may support Frequency Division Duplex (FDD) scheme and Time Division Duplex (TDD) scheme. In the FDD scheme, an uplink transmission and a downlink transmission may be simultaneously performed in a cell because a carrier frequency for an uplink (UL) transmission is different from a carrier frequency for a downlink (DL) transmission exists. In the TDD scheme, with respect to one cell, an uplink transmission and a downlink transmission are distinguished from each other based on different time slots. In the TDD scheme, a base station and a user equipment perform switching operations between a transmission mode and a reception mode because the same carrier is used for both an uplink transmission and a downlink transmission. In the TDD scheme, a Special Subframe may be added to provide a guard time for switching between the transmission mode and the reception mode. The Special Subframe may include Downlink Pilot Time Slot (DwPTS), Guard Period (GP), and Uplink Pilot Time Slot (UpPTS). According to the TDD scheme, resource amounts for the uplink transmission and resource amounts for the downlink transmission may be asymmetrically assigned through various uplink (UL)-downlink (DL) configurations.
Currently, remaining frequency resources are scarce and various technologies have been utilized in wide frequency bands because of the frequency resource scarcity. For this reason, in order to provide a wideband bandwidth for supporting higher data-rate requirements, each of scattered bands has been configured to satisfy basic requirements to operate an independent system and a carrier aggregation (CA) scheme, which aggregates various frequency bands into one system, has been adopted. Here, each frequency band or carrier capable of an independent operation may be defined as a component carrier (CC). According to the adoption of the carrier aggregation system, ACK/NACK signals corresponding to a plurality of component carriers (CCs) need to be transmitted.
Recently, a Time Division Duplex (TDD)-Frequency Division Duplex (FDD) Carrier Aggregation (CA) that supports a CA and/or dual connectivity of a FDD band (or carrier) and a TDD band (or carrier) has been considered. The TDD-FDD CA is referred to as a TDD-FDD joint operation. However, when it is assumed that a plurality of serving cells that are aggregated by the CA exist, and a first serving cell is configured as TDD and a second serving cell is configured as FDD, there may be difficulty in transmitting a HARQ ACK/NACK for downlink (DL) transmission on all subframes of the second serving cell according to the TDD-FDD CA. For example, in a circumstance that allows only a control channel of the first serving cell (that is, a Physical Uplink Control Channel (PUCCH)) for transmitting a HARQ ACK/NACK of the second serving cell, a large number of DL subframes of the second serving cell may exist in association with a single UL subframe of the first serving cell. Therefore, there is desire for a method of effectively transmitting a HARQ ACK/NACK, for the TDD-FDD CA.